1. Field of the Invention
The present invention relates to a method of manufacturing a near-field light generator for use in thermally-assisted magnetic recording where a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head section including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head section including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating the near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near a medium facing surface of the slider. The waveguide includes a core through which light propagates, and a cladding provided around the core.
The plasmon generator has a front end face located in the medium facing surface. The front end face generates near-field light. Surface plasmons are excited on the plasmon generator and propagate along the surface of the plasmon generator to reach the front end face. As a result, the surface plasmons concentrate at the front end face, and near-field light is generated from the front end face based on the surface plasmons.
U.S. Patent Application Publication No. 2011/0170381 A1 discloses a technology in which the surface of the core of the waveguide and the surface of a metallic structure (plasmon generator) are arranged to face each other with a gap therebetween, and evanescent light that occurs at the surface of the core based on the light propagating through the core is used to excite surface plasmons on the metallic structure, so that near-field light is generated based on the excited surface plasmons.
In order to reduce the track width of a recording medium for higher recording density, it is required to reduce the near-field light in spot diameter at the recording medium. To achieve this, it is required to reduce the width and height of the front end face of the plasmon generator. Note that the width of the front end face refers to the dimension of the front end face in the track width direction, and the height of the front end face refers to the dimension of the front end face in the direction in which the tracks extend. The width and height of the front end face are both preferably 50 nm or smaller.
Here, a device that includes a waveguide and a plasmon generator and generates near-field light will be referred to as a near-field light generator. For the structure of the near-field light generator, the structure in which the plasmon generator is disposed above the top surface of the core of the waveguide, as disclosed in U.S. Patent Application Publication No. 2011/0170381 A1, is conceivable.
The near-field light generator having the above-described structure can be manufactured by the following method, for example. A first cladding layer to underlie the core is formed first. Then, the core is formed on the first cladding layer. The core has a bottom surface, a top surface opposite thereto, and two side surfaces connecting the top and bottom surfaces to each other. Next, a second cladding layer is formed to cover the first cladding layer and the core. The second cladding layer is then polished so that the top surface of the core is exposed. The second cladding layer contacts the two side surfaces of the core. Then, a third cladding layer is formed over the core and the second cladding layer. Next, a plasmon generator, and a dielectric layer lying therearound are formed on the third cladding layer. The plasmon generator and the dielectric layer are then polished to determine the thickness of the plasmon generator. For example, chemical mechanical polishing (hereinafter referred to as CMP) is used for the polishing of the second cladding layer and the polishing of the plasmon generator and the dielectric layer. The first to third cladding layers constitute the cladding.
The thickness of the plasmon generator has an influence on its performance and therefore must be controlled accurately. The above-described manufacturing method for the near-field light generator, however, has a problem that hampers accurate control of the thickness of the plasmon generator. The problem will now be described.
In the above-described manufacturing method for the near-field light generator, the thickness of the plasmon generator is determined in the step of polishing the plasmon generator and the dielectric layer. In order to control the thickness of the plasmon generator accurately, the flatness of the top surface of each of the plasmon generator and the dielectric layer after the step of polishing the plasmon generator and the dielectric layer must be improved. The flatness of the top surface of each of the plasmon generator and the dielectric layer is affected by the shape of the top surface of each of the core and the second cladding layer. Therefore, to control the thickness of the plasmon generator accurately, the flatness of the top surface of each of the core and the second cladding layer after the step of polishing the second cladding layer must be improved.
In the above-described manufacturing method for the near-field light generator, however, the difference between the materials used for the core and the second cladding layer generates a difference in level between the top surface of the core and the top surface of the second cladding layer or causes the top surface of the core to curve, so that the flatness of the top surface of each of the core and the second cladding layer is low after the step of polishing the second cladding layer. As a result, the flatness of the top surface of each of the plasmon generator and the dielectric layer is also low after the step of polishing the plasmon generator and the dielectric layer.